Electrochemical cell holder and stack

ABSTRACT

A fuel cell stack made of a plurality of cell units stacked and operatively connected at one end thereof. Each of the units includes a holder having at least one cell, typically provided as an SOFC membrane, to produce an electric current when fuel and oxidant are present as the result of an electrochemical reaction.

This application is a divisional application of U.S. Ser. No. 12/094,156filed Jun. 9, 2008, which is a §371 of PCT/US2006/045199 filed Nov. 22,2006, and claims priority from U.S. Provisional Patent Application No.60/739,229 filed Nov. 23, 2005, which are hereby incorporated byreference in their entireties.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made during work supported by U.S. Department ofEnergy under Contract No. DE-AC03-76SF00098. The government has certainrights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of electrochemical devices,and more specifically solid oxide fuel cells (SOFCs), electrolyticoxygen generators, and electrolyzers.

2. Background

Steadily increasing demand for power, increasing fuel costs, and theatmospheric build up of greenhouse and other combustion gases hasspurred the development of alternative energy sources for the productionof electricity. Fuel cells hold the promise of an efficient, lowpollution and environmentally friendly technology for generatingelectricity from a wide variety of fuels. However, the present cost ofelectrical energy production from fuel cells is several times higherthan the cost of the same electrical production from commercialtechnologies. The high cost of capitalization and operation per kilowattof electricity produced has delayed the commercial introduction of fuelcell generation systems.

Solid oxide fuel cells offer the potential of high efficiency combinedwith fuel flexibility. Considerable progress is being made in raisingthe performance and therefore lowering the per unit cost of solid oxidefuel cells, and as an example, one of the present inventors was thefirst to demonstrate that power densities of as much as 2 W/cm² could beobtained for supported thin-film yttria-stabilized zirconia (YSZ) solidoxide fuel cells, at 800° C., see S. de Souza, S. J. Visco, and L. C. DeJonghe, “Reduced-temperature solid oxide fuel cell based on YSZthin-film electrolyte,” J. Electrochem. Soc., 144, L35-L37 (1997), thecontents of which are hereby incorporated in their entirety for allpurposes. While this result was encouraging, further reductions intemperature to below 800° C. would aid in lowering the system cost. Suchreduction in operating temperature on the one hand makes the use ofmetallic interconnects and support electrodes possible, allowing forcost reduction, and on the other hand allows new ways of configuringfuel cells so that current can be collected with minimal resistive loss.

A conventional fuel cell is an electrochemical device that convertschemical energy from a chemical reaction with the fuel directly intoelectrical energy. Electricity is generated in a fuel cell through theelectrochemical reaction that occurs between a fuel (typically hydrogenproduced from partially oxidized or reformed hydrocarbons such asmethanol, ethanol, propane, butane or methane) and an oxidant (typicallyoxygen in air). This net electrochemical reaction involves chargetransfer steps that occur at the interface between theionically-conductive electrolyte membrane, the electronically-conductiveelectrode and the vapor phase of the fuel or oxygen. Other types of fuelcells known in the prior art include molten carbonate fuel cells,phosphoric acid fuel cells, alkaline fuel cells, and proton exchangemembrane fuel cells. Because fuel cells rely on electrochemical ratherthan thermo-mechanical processes in the conversion of fuel intoelectricity, the fuel cell is not limited by the Carnot efficiencyexperienced by conventional mechanical generators.

Solid oxide fuel cells are all solid devices that offer the potential ofa high volumetric power density combined with fuel flexibility. FIG. 1illustrates a cross section of a fuel cell, in particular a solid oxidefuel cell (SOFC) (10). The cell consists of two electrodes, an anode(16) and a cathode (18) separated by an electrolyte (17). In thisexample, a nickel-yttria-stabilized zirconia cermet (Ni/YSZ) is thematerial used for the anode (16). Lanthanum strontium maganite (LSM) isthe material used for the cathode (18) and yttria-stabilized zirconia(YSZ) is used for the electrolyte. Many other combinations of materialsmay be used to construct a SOFC. Fuel (11), such as H₂, CO, and/or CH₄(the present invention may be used with other fuels) is supplied to theanode (16), where it is oxidized by oxygen ions (O²⁻) from theelectrolyte (17), which releases electrons to the external circuit. Onthe cathode (18) an oxidant such as O₂ or air is fed to the cathode,where it supplies the oxygen ions from the electrolyte by acceptingelectrons from the external circuit. The electrolyte (17) conducts theseions between the electrodes, maintaining overall electrical chargebalance. The flow of electrons in the external circuit provides power(15), which may be siphoned off from the external circuit for otheruses. Reaction products (12) are exhausted off the device. Excess air(14) may be passed through the device.

In conventional SOFCs, the electrolytes are typically formed fromceramic materials, since ceramics are able to withstand the hightemperatures at which the devices are operated. For example, SOFCs areconventionally operated at about 850° C. to 1000° C. Also, typical solidstate ionic devices such as SOFCs have a structural element on to whichthe SOFC is built. In conventional planar SOFCs the structural elementis a thick (100-500 μm) solid electrolyte plate such as yttriastabilized zirconia (YSZ); the porous electrodes are then screen printedonto the electrolyte.

In the case of a typical solid oxide fuel cell, the anode is exposed tofuel and the cathode is exposed to an oxidant in separate closed systemsto avoid any mixing of the fuel and oxidants due to the exothermicreactions that can take place with hydrogen fuel.

The electrolyte membrane is normally composed of a ceramic oxygen ionconductor in solid oxide fuel cell applications. In otherimplementations, such as gas separation devices, the solid membrane maybe composed of a mixed ionic electronic conducting material (“MIEC”).The porous anode may be a layer of a ceramic, a metal or, most commonly,a ceramic-metal composite (“cermet”) that is in contact with theelectrolyte membrane on the fuel side of the cell. The porous cathode istypically a layer of a mixed ionically and electronically-conductive(MIEC) metal oxide or a mixture of an electronically conductive metaloxide (or MIEC metal oxide) and an ionically conductive metal oxide.

Solid oxide fuel cells normally operate at temperatures between about850° C. and about 1000° C. to maximize the ionic conductivity of theelectrolyte membrane. At appropriate temperatures the oxygen ions easilymigrate through the crystal lattice of the electrolyte. However, mostmetals are not stable at the high operating temperatures and oxidizingenvironment of conventional fuel cells and become converted to brittlemetal oxides. Accordingly, solid-state electrochemical devices haveconventionally been constructed of heat-tolerant ceramic materials.However, these materials tend to be expensive and still have a limitedlife due to their brittle nature. In addition, the materials used musthave certain chemical, thermal and physical characteristics to avoiddelamination due to thermal stresses, fuel or oxidant infiltrationacross the electrolyte and similar problems during the production andoperation of the cells.

Since each SOFC generates a relatively small voltage, several SOFCs maybe associated to increase the capacity of the system. Such arrays orstacks generally have a tubular or planar design. FIG. 2 illustrates abasic planar design for a solid state electrochemical device, forexample a solid oxide fuel cell (SOFC). The cell (10) includes an anode16 (the “fuel (fuel 11) electrode”) and a cathode (18) (the “air,(oxidant 13) electrode”) and a solid electrolyte (17) separating the twoelectrodes. An interconnect (19) separates the fuel and the oxidant andelectrically connects one cell to another in series. Typically amultitude of cells are “stacked” to make a “stack”. In reality, there isno space between the stacks as shown in FIG. 2 and one set ofanode/electrolyte/cathode/interconnect is in contact with the next.

Planar designs, however, are generally recognized as having significantsafety and reliability concerns due to the complexity of sealing of theunits and manifolding a planar stack. As shown in FIG. 2 the cells andinterconnect are in contact with each other at various points. Such anassembly requires high flatness tolerances in order to avoid unevencontact pressure and inhomogeneous stress distribution. Inhomogeneousstress increases the risk of cell failure during assembly or operation.The high flatness tolerance of the cells increases the production cost.Also to avoid stress due to temperature gradients across the cell theymust be heated and cooled very slowly. The slow heat up results inwasted fuel and a subsequent decrease in efficiency for applicationsrequiring a large number of on/off cycles.

Conventional stacks of planar fuel cells operated at the highertemperature of approximately 850-1000° C. have relatively thickelectrolyte layers compared to the porous anode and cathode layersapplied to either side of the electrolyte and provides structuralsupport to the cell. However, in order to reduce the operatingtemperature to less than 800° C., the thickness of the electrolyte layerhas been reduced from more than 50-500 microns to approximately 5-50microns. The thin electrolyte layer in this configuration is not a loadbearing layer. Rather, the relatively weak porous anode and cathodelayers must bear the load for the cell. Stacks of planar fuel cellssupported by weak anodes or cathodes may be prone to collapse under theload, e.g., during stack construction or thermal cycling. Reducing themechanical stress of the cells helps avoid cell failure.

In addition, SOFC stacks should have a short startup time and possessstability during thermal cycling in certain applications, includingauxiliary power unit (APU) and portable power applications. Availableprior art stacks can tolerate ˜70° C./min heating procedure and it takes˜10 minutes to reach 700° C., but the stack stability over rapid thermalcycling remains unknown.

Prior art planar stacks suffer from the fact that all four sides (ifrectangular) are coupled to each other and the cell membrane and thecell membrane is coupled to the interconnect. A multitude of cells arestacked together and therefore all mechanically coupled. Thisarrangement induces thermal and mechanical stresses during operationthat cause various failures within cells in the stack, decreasingperformance and lifetime of the device. In one attempt to solve theproblems of the prior art U.S. Published application no. 20030096147 A1,published May 22, 2003, the contents of which are hereby incorporated byreference in its entirety for all purposes, discloses solid oxide fuelcell assemblies having packet elements having an enclosed interiorformed in part by one or more compliant solid oxide sheet sections witha plurality of anodes disposed within the enclosed interior.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrochemical cellholder and stack that alleviates chemical and mechanical stressesgenerally associated with available planar stacks; that demonstrate arelatively brief startup time, provide stability during thermal andmechanical shock, and exhibit improved electrochemical performance. Itis also an object of the invention to provide improved cell units foruse in the manufacture of cell stacks. It is also an object of theinvention to provide a cell holder that can utilize cell membranes oflower flatness tolerances and thereby reduce cost.

These objects and others are achieved according to the presentinvention, which relates in part to a cell stack made of a plurality ofcell units stacked and operatively connected. Each of the units includesa holder having at least one cell, typically provided as an SOFCmembrane, to produce an electric current when fuel and oxidant arepresent as the result of an electrochemical reaction. The individualunits forming the stacks are made of a cell membrane holder with one ormore cell membranes mounted thereto. The cell membrane has an anode, acathode, and an electrolyte, typically arranged between the anode andcathode.

The holder preferably includes an opening (window) for mounting the cellmembrane, an electrical contacting portion adjacent to the anode, anelectrically insulating portion to electronically isolate the cathode,and a gas outlet and/or inlet adjacent to the window.

The holders may be made of a single part, of two parts, or three ormore.

In a preferred embodiment the holder is made of three parts, a frontwall plate, a back wall plate, and a spacer positioned therebetween. Atleast one of the front and/or back wall plates has an opening therein(window) to which the cell membrane is mounted, preferably with theanode spaced apart but facing inward toward the opposite plate when theholder is assembled with the spacer positioned between the front andback plates. At least one of the front wall or back wall has a gasoutlet and/or gas inlet for providing fuel, e.g., hydrogen gas,hydrocarbon gas, or reformed hydrocarbons, to the interior electrodes,typically the anodes. An oxygen generator only requires a gas outlet.The spacer also has a window that generally corresponds to the windowsof the front and/or back wall plates and communicates with the fuelinlet and outlet so that fuel can reach and contact the inner electrodesand the electrochemical reaction can take place to generate electricity.The back wall plate, the front wall plate, and the spacer are alignedand physically connected, e.g., by welding, to form the holder. Thesecomponents may be in electrically conductive contact with one another,but may not be.

A portion of the holder must be made of an electrically conductivematerial and must be in electric contact with the anodes of the cellmembranes mounted on the front and/or back walls of the holder. In oneembodiment the front and back wall plates are in electrically conductivecontact with both the anodes and the spacer. In other embodiments, thefront and back wall plates are not electrically conductive, or are madeof an electrically conductive material coated with a non-conductivematerial on at least a portion thereof and an electrically conductivespacer is in contact with the anodes.

The outer electrodes of the cell membrane, typically the cathodes, faceoutwardly towards the environment such that they can be exposed toambient air or another oxidant. These electrodes are electronicallyisolated from the anodes such that the only current flowing between theelectrodes is predominately in the form of ions and through theelectrolyte.

The electrolyte is positioned between the anode and cathode of the cell.

The cell membrane may be affixed to a receiving portion of the back orfront wall proximate to the respective window by any suitable means,e.g., a sealant. The seal may be conductive or insulating, or layers ofeach may be provided. Each cell membrane may be affixed or adhered tothe front and back plates using different adhesives. In anotherpreferred embodiment, the holder includes a front and back wall havingwindows and fuel flow channels defined therein, but no spacer (two-partconstruction).

In another preferred embodiment, the holder is a single plate.

In preferred embodiments, the holder is made entirely of stainless steelbut may be made of any suitable material fit for the intended purposeprovided some portion is made of an electrically conductive material,and may also be annealed.

A particularly preferred embodiment relates to a holder for a cell stackassembly having an opening for mounting the cell membrane, and anelectrical contacting portion for contacting an anode, wherein the gasmanifold is positioned at a periphery of the holder which will be inelectric contact with an another holder.

Two or more units, that is, an assembly made of the holder with at leastone cell membrane mounted thereto, may be stacked to increase energyoutput by operatively connecting the two or more units to each other.The units, individual or stacked, will also preferably be electricallyconnected to an outer circuit through which electrons produced viaelectrochemical reaction at the electrodes will flow.

The stacks may be formed in any operative arrangement, and mayoptionally be arranged in a housing. A preferred embodiment is directedto a cell stack assembly made of a plurality of planar cell units,wherein each of the cell units include a cell holder and at least onecell that are electrically connected. The at least one cell comprises ananode, a cathode, and an electrolyte; and the plurality of cell unitsare connected at a portion of a periphery of the cell holder of eachcell unit. In this embodiment, the plurality of cell units areelectrically, mechanically and/or connected at a portion of a peripheryof the cell holder of each cell unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art solid oxide fuel cell (SOFC) operation diagram.

FIG. 2 shows a prior art solid oxide fuel cell in planar arrangementconsisting of multiple cells.

FIG. 3A is a schematic of a cell holder in accordance with oneembodiment of the present invention.

FIG. 3B shows a cell mounted to a holder with a seal according to anembodiment of the present invention.

FIG. 4A shows the components of a holder according to one embodiment ofthe present invention.

FIG. 4B is a photograph of a front piece, spacer and end piece of aholder made of stainless steel prepared in accordance with oneembodiment of the present invention.

FIG. 5 is a photograph of a smaller (left) and a larger (right) chipcell holder in accordance with the present invention; the larger holderhas gas flow directors (baffles) (99).

FIG. 6A shows a cross section of a unit comprising two cells inaccordance with one embodiment of the present invention.

FIG. 6B shows a side view of the unit of FIG. 6A.

FIG. 6C shows a cross section of unit comprising two cells in accordancewith another embodiment of the present invention.

FIG. 7 depicts a cell attached to a holder by brazing.

FIG. 8 depicts a cell attached to a holder by a ceramic, glass or glassceramic cell on an end with an electronically conductive paste providedto provide electric contact between the holder and the cell.

FIG. 9 depicts a cell attached to a holder similar to that shown in FIG.8 but providing a glass topcoat gastight seal.

FIG. 10A is a photograph of an assembled unit in accordance with theinvention with silver mesh.

FIG. 10B is a photograph of an assembled unit in accordance with theinvention with silver mesh and Ag lead wires.

FIG. 11 is a performance curve for a unit prepared according to oneembodiment of the present invention.

FIG. 12 is a graph showing variation of open circuit voltage (OCV) for aunit in accordance with the invention.

FIG. 13 is a graph showing the temperature variation and its changingrate during thermal shock treatment.

FIG. 14 is a graph showing OCV measured at 708° C. vs. the number ofshock cycles for a chipcell in accordance with the present invention.

FIG. 15 is a photograph of a two-unit (chipcell) butterfly stack inaccordance with the present invention.

FIG. 16 is a scanning electron micrograph (SEM) of a brazed joint of aholder with insulator in accordance with the present invention.

FIG. 17 is a graph showing the variation of the OCV of a two-unitbutterfly stack in accordance with the present invention.

FIG. 18A shows gas flow in a solid oxide fuel cell stack based on acombination of units connected in one embodiment of the presentinvention.

FIG. 18B is an alternative embodiment showing gas flow through severalcombined units connected in one embodiment of the invention.

FIG. 19 shows a unit having a plurality of cells, in this case fourcells, in accordance with one embodiment of the present invention.

FIG. 20 shows a unit having a plurality of cells in accordance with oneembodiment of the present invention.

FIG. 21 depicts an embodiment of the invention with holes to accommodatebolts for stacking.

FIG. 22 depicts a circular shaped embodiment of the invention.

FIG. 23 depicts an alternative circular shaped embodiment of theinvention

FIG. 24 shows stacked units bonded with insulating material at one end.

FIG. 25 shows stacked units bonded at two ends.

FIG. 26 shows a butterfly stack arrangement of a combination of units inaccordance with an embodiment of the present invention.

FIG. 27 shows a ladder arrangement of units connected in one embodimentof the present invention.

FIG. 28 shows a two-piece holder according to an embodiment of thepresent invention.

FIG. 29 shows a one-piece holder according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By “cell” it is meant an electrochemical cell. In one embodiment of thepresent invention this means at least two electrodes with an electrolytein between. This may also be termed “cell membrane” or “SOFC cell”herein.

By “holder” it is meant a structure that houses the cell and providegas-flow paths and may or may not be electrically conductive.

By “unit” it is meant at least one “cell” or “cell membrane” in a holderin accordance with the invention. The term “unit” may include otherstructural elements such as housing, interconnect wires, pumps and otherequipment for operation of fuel cell stacks. This may also be termed“chip” or “chipcell” herein.

By “stack” as it is used here it is meant a plurality of “units”connected, e.g., in a horizontal or vertical configuration. Theelectrical connections can be in series or parallel or a combination ofseries and parallel.

By “housing” it is meant some structure that encloses a unit or cell orstack. The term “housing” is used generically herein, and does not referto one specific shape or structure but to those structures that enclosethe cell, chip and/or unit. An “endwall” may be part of the housing orthese terms may be used interchangeably.

A preferred holder of the invention is illustrated in FIG. 3A. A window(21) or opening is provided in each front and back wall plates (22, 23)to accommodate a cell membrane (24), wherein the anodes preferably faceeach other in an inward direction but are spaced apart by a spacer (25)positioned between front and back wall plates (22, 23) which forms afuel cavity (32) between electrodes, typically the anodes which servesas a fuel passageway, which may include a separate fuel passageway. Thisarrangement forms an anode chamber in the interior of the unit. Frontand back wall plates (22, 23) may also have a receiving region (26)adjacent the window to which the cell membrane (24) may be affixed by asealant.

Referring to FIGS. 3A and 4A, it can be seen that spacer (25) has acutout window (21 c) therein generally corresponding to the windows (21a, 21 b) of the front and back wall plates (22, 23), such that theanodes face each other but are spaced apart with a fuel cavitytherebetween. Spacer (25) can be electrically conductive and in electriccontact with each of the anodes (30) of the cell membranes. In FIG. 3B,the anodes (30) are electrically contacted with the front and back walls(22, 23), which in turn are in electric contact with the spacer (25).Thus, the anodes (30) are indirectly in electric contact with spacer(25). Alternatively, front and back walls (22, 23) may be electricallyinsulated from the anodes and spacer (25), in which case the anode is indirect electrical contact only with spacer (25) of the holder.

Fuel may pass through fuel cavity (32) and, if provided, the fuelpassageway during operation of the device. It is understood that inaccordance with one embodiment of the present invention fuel cavity (32)is sealed to the outside atmosphere and the only material incommunication with the anodes (30) is the fuel that is provided. In oneembodiment, fuel cavity (32) includes a fuel passageway that comprisestubing or other conduit means for supplying fuel to the anodes (30). Thefuel passageway must have means of contacting the fuel with the anodes(30). Note that each cell membrane (24) is positioned adjacent to oneanother such that electrodes of one type, e.g. the anodes (30) arefacing inward toward one another and electrodes of the other type areeach facing outward, cathodes (50). Layered on the outside of each anode(30 a) and (30 b) is an electrolyte (60) and (62) respectfully. Layeredon electrolyte (60) is a cathode (50).

A current collector (70) such as a stainless steel or silver mesh,conductive paste, or porous alloy sheet is preferably in electriccontact with the cathode (50) as shown in FIG. 10B. The currentcollector may be affixed to the cathode with a suitable material (71) ormechanical fastener, which is preferably electrically conductive. Thecurrent collector may be affixed to the cell and/or holder with anelectrically insulating adhesive as shown in FIG. 10 A. Lead wires (72,76) may be provided as the current collector attached to the currentcollectors as shown in FIG. 10B. In some embodiments the currentcollector is the same material as the leads or wires or interconnect(72) and (76), e.g., silver. In other embodiments the current collectormay be a silver paste and the leads, wires or interconnects (72) and(76) would then comprise a wire or mesh or other material suitable forelectrical use. Any suitable materials may be used for the lead wires,current collector and seals or fastener. For example, coated metalscreens are also suitable. U.S. Pat. No. 6,740,441 describes a coatedcurrent collection device capable of collecting current in anelectrochemical cell comprising a coated metal screen, mesh or felt.

The invention contemplates that a multitude of units may be electricallyconnected. It is understood that the shape of the connectors is only oneembodiment and any connector will be suitable so long as the propercurrent flow path is preserved.

Cells may be mounted in the holder receiving region in the vicinity ofthe window (21) of the front and back walls (22, 23) using anelectrically conductive seal (78) or a combination of conductive andinsulating seals (77) as shown in FIGS. 7 to 9. The seal (71) has atleast three functions. A first function is to seal the externalatmosphere (typically air) from the internal atmosphere (typicallyH₂+H₂O or reformed hydrocarbon gas mixture). A second function is tobond the cell to the holder. The third function is to form an electricalcontact between the anode and the holder. The third function may also bereplaced by forming an electrical contact between the gas flow channelsand the cell membrane. Three possible seals are shown in FIGS. 7-9;these are examples and not meant to be limiting.

Referring to FIGS. 3A and 3B, fuel, typically hydrogen containing gas,is fed through a fuel inlet (31) in a front or back wall (22, 23) ofholder (20), and is received into the fuel cavity (32) between anodes(30) via fuel receiving means in spacer (25) which communicate with fuelinlet (31) via communication region (33). The fuel then undergoeselectrochemical oxidation on the inwardly facing anodes (30) beforeexhausting out via exhaust outlet (34) provided in the front or backwall (22, 23) of the holder 20. Fuel inlet (31) and exhaust outlet (34)may be positioned in any variety of ways provided they provide thenecessary flow of fuel and properly exhaust spent fuel or anybyproducts.

Holder (20) is preferably made of stainless steel as shown in FIG. 4B,but ceramics, alloys, and composites may also be used to form theholder. Stainless steel is particularly preferred and is electricallyconductive. Typical alloys for use in accordance with the inventioninclude ferritic steel with Cr contents between 12-30 wt % such as AISI410L, 430L, 434L, 446, and Ebrite®, but these are exemplary and notlimiting. Nickel based alloys are well known in the art and can also beused.

Spacer (25) is preferably made of an electrically conductive material,but the front and back walls may be electrically conductive ornon-conductive or form an electronically insulating layer such as Al₂O₃that forms on Al containing alloys such as FeCrAlY.

As shown in FIG. 5, the holder may have projections (48) or other flowdirecting portions to direct or control the flow of fuel as it travelsthrough fuel passageway (32).

It is known in the art that coating the stainless steel can reduce theoxidation rate and decrease the chromium vaporization in moist air (see,for example, “Protective coating on stainless steel interconnect forSOFCs: oxidation kinetics and electrical properties” in Solid StateIonics, Volume 176, Issues 5-6, 14 Feb. 2005, Pages 425-433 by XuanChen, Peggy Y. Hou, Craig P. Jacobson, Steven J. Visco and Lutgard C. DeJonghe). Such coatings are contemplated for the chipcell holderdescribed herein.

The dimensions of the holder may vary with the desired application andthe shape of the window frame may be of any suitable shape a square orrectangular shape is shown in FIG. 3A, and circular embodiments aredepicted in FIGS. 22 and 23.

Holder (20) can be manufactured through any viable techniques such asextrusion, molding, casting, machining, stamping, punching, sintering,brazing, bonding or any combination of these or other methods known inthe art, and typically will depend on the material chosen to make theholder. The front wall (22), back wall (23) and spacer (25) arepositioned such that the windows and any fuel inlets (31) or outlets(34) are functionally aligned, and are physically connected by any meansknown in the art, e.g., by welding. Three parts are shown in FIGS. 4Aand 4B, however fewer or more parts may be used to construct the holder.

Any number of even and odd numbers of membranes may be used on each sideof the unit. It is preferred that the number of the cell membranes oneach side of the unit is the same, and a plurality of cells may beprovided in respective windows, as shown in FIG. 19-20. In FIG. 19 andFIG. 20 it can be see that the holder components have windows therein toreceive a plurality of the SOFC cells, and any number of cells may beprovided.

Any suitable cell membrane, whether specially manufactured orcommercially available, may be used in accordance with the presentinvention. Selection of the cell will depend on a number of factorsknown to those skilled in the art; e.g., certain cathode, anode orelectrolyte combinations may be preferred in certain applications overothers. While not wishing to be bound by any particular theory orprinciple, operation of a SOFC in one embodiment of the inventionproceeds as follows. An oxidant, preferably air which provides O₂ issupplied. Fuel, preferably partially oxidized or reformed hydrocarbonsis supplied to be in contact with the anode through a fuel channel.Electrons supplied to the cathode will reduce the oxygen toO²⁻(O₂+4e−→2O²⁻). Oxygen ions will be ionically transported across eachelectrolyte to the anode. When the oxygen ions reach the fuel at theanode they oxidize the hydrogen to H₂O and the CO to CO₂. In doing sothey release electrons, and if the anode and cathode are connected to anexternal circuit this flow of electrons is seen as a dc current.Electric power is drawn from the unit or stacked unit. This processcontinues as long as fuel and air are supplied to the cell.

Electrochemical devices such as fuel cells, electrolytic oxygengenerators, and electrolyzers have an electrolyte with and anode on oneside and a cathode on the opposite side. The cells may be electrolytesupported where the mechanical strength of the cell is due to anelectrolyte between 50-1000 μm thick. Thin film electrolytes (<50microns thick) require a support that is usually the anode (for exampleNi-YSZ) or cathode (such as LSM). Metal or cermet support structuressuch as described in U.S. Pat. No. 6,605,316 can also be used. Thechipcell design of the present invention can utilize any of these cells.Well known electrolytes include: yttria stabilized zirconia (YSZ) with 3mol %, 8 mol % or 10 mol % yttria; scandia stabilized zirconia (SSZ);doped ceria such as gadolinia or samaria doped ceria (GDC or SDC); anddoped lanthanum gallate such as strontium and magnesium doped lanthanumgallate (LSGM). These are merely examples and the invention is not solimited.

The electrodes in accordance with the present invention may comprise anysuitable materials, e.g., a porous ferritic stainless steel, (forexample in Steven J. Visco, Craig P. Jacobson, Igor Villareal, AndyLeming, Yuriy Matus and Lutgard C. De Jonghe, “Development of Low-CostAlloy Supported SOFCs”, Proc. ECS meeting, Paris, May 2003, the contentof which are hereby incorporated by reference in its entirety for allpurposes) about 0.4 mm thick, activated by incorporation of a Ni/ceriadispersion such as described in U.S. Pat. No. 6,682,842. Additionally,stable increased catalytic activity may be obtained by post-infiltrationwith compounds that form nano-scale catalyst particles near or at theelectrolyte/electrode interface, as in Keiji Yamahara, Craig P.Jacobson, Steven J. Visco, Lutgard C. De Jonghe, “High-Performance ThinFilm SOFCs for Reduced Temperature”, Proceedings SSI 14, Monterey,Calif., 2003, the contents of which are hereby incorporated by referencein their entirety for all purposes. The ferritic steels have thermalexpansion coefficients that can approximately match those of the ceramicelectrolyte, thereby avoiding thermal stresses and allowing for highheating rates and thermal cycling. The cathode current collection may befacilitated by a supporting stainless steel mesh or Ag mesh that isincorporated with the cathode. The supported thin film electrolyte maybe produced by colloidal processing and co-firing as disclosed in U.S.Pat. No. 6,458,170. Materials for the electrolyte and electrodes areknown in the art, and these and others yet to be discovered may be usedin accordance with the present invention. Preferred are solidelectrolytes include samaria-doped ceria (SDC), gadolinia doped ceria(GDC), yttria stabilized zirconia (YSZ), scandia stabilized zirconia,and La_(0.9)Sr_(0.1)Ga_(0.8)Mg_(0.2)O_(3-δ) (LSGM). Previous work by thepresent inventors has demonstrated obtainable area specific power inexcess of 500 mW/cm² between 600 and 650° C., for a cell membrane with aLanthanum-Strontium-Cobalt-Nickel oxide (LSCN)/samaria-doped ceria (SDC)composite cathode, and a Ni/SDC composite anode (C. P. Jacobson, S. J.Visco, and L. C. De Jonghe, “Thin-Film Solid Oxide Fuel Cells forIntermediate Temperature (500-800° C.) Operation”, Proc. Of theProcessing and Characterization of Electrochemical Materials andDevices, Apr. 25-28, 1999, The American Ceramic Society).

The present invention contemplates that a plurality of units (1) may becoupled via a connector to create a stack of units. The construction ofthe unit and the stack minimizes stress transfer to the cells. Thestacked units are operatively connected to one another so that fuel,oxidant, and any exhaust gases may flow as required to operate the cell.The units are also typically electrically connected to each other and toa collector circuit through which electrons produced duringelectrochemical processes in the device may flow and used for otherapplications.

Current flows to the cathode as electrons, then through the electrolyteas ions, and then from the anode to the holder as electrons. Electricalcontact needs to be made between the holder and therefore anode of oneunit to the cathode of the adjacent unit. As shown in FIG. 6 aninsulator (100) can be positioned between the two units to preventunwanted electrical contact between the anodes of adjacent units.Electrical contact between the holder of unit and the cathode currentcollector of the adjacent unit is made by a conducting material (101).This may be a paste or wire or mesh or sheet or combination of these.FIG. 6B shows a conductor (101) in contact with both cathode currentcollectors (70) but insulated from the holder by an insulator (100). Theconductor (101) makes electrical contact with the adjacent cell. Asshown in FIG. 16 a ceramic material such as Macor® may be used as theinsulator. Other materials include 3 mol % YSZ, Al₂O₃, MgO, and mica(many more insulators are known in the art and can be used here). FIGS.10A and 10B show a silver mesh current collector electronicallyinsulated from the holder by the seal. Silver wire leads bring currentto the electrode in this configuration.

While units can be connected in electrical series, they are not limitedto then and the stack can have some units in electrical series andothers connected in parallel. The flexibility in the unit architectureand the series assemblies can be readily envisioned to lead tocombinations that range from a few Watts to 10s of kilowatts, in highlycompact power generating devices. Anode gas flow can be in a cascadearrangement to increase stack efficiency. If connected with another unitat connectors there is preferably a space defined on one side by thecathode and on the other side by the cathode of the other coupled unit.The space may communicate via opening and with the cathode for air inletand exhaust such that the air will be exposed to the cathode. Theseopenings may be exposed to ambient air or have some external supply ofan oxidant gas.

Stacked units may be housed in a housing which is preferably defined byan endplate. An air intake may be provided for each cell unit (1) sothat each cathode is exposed to the atmosphere or other oxygen source.Alternatively, cathode may be exposed to ambient air. Endplate orhousing may have any structure depending on the desired end use, so longas there is communication means for supplying air to the cathodes. Thiscommunication means may just be that there is no end plate, housing andthe cathode is exposed to ambient air. Air exhaust is provided in thehousing for exhaust air.

FIG. 18A and FIG. 18B show a stacked arrangement of cells (1 a-1 c)separated by insulator (40). Alternate inlet and exhaust arrangementsare also shown (compare FIGS. 18A and 18B. Separate inlets and exhaustmay be provided per each unit (1), but the fuel inlet and exhaust may bedesigned to flow through all of the stacked units.

FIGS. 16 and 26 shows an alternative “butterfly” arrangement of two ormore units (1 a, 1 b, etc.) separated by an insulator but havingcommunicating fuel inlets and exhaust outlets. Such butterflyarrangements allow the SOFC membranes to be applied after holders (10)are operatively connected, e.g., by brazing which decreases the risk ofdamaging the SOFC cell during brazing of the individual holders to eachother.

FIG. 27 shows an alternative ladder arrangement of a plurality ofchipcells (units) according to the invention. The units (1 a, 1 b, 1 c,etc.) are connected to ladder frames 46, with separate ladders beingoperatively held together with ladder frame connectors 47. A ladderframe end portion on 48 may be provided at an end of a ladder frame, andmay be made of electrically conductive or non-conductive material.

The invention contemplates that the structures described herein are tobe used as oxygen generators as well as SOFC devices, wherein current orvoltage is supplied to the device and oxygen is produced at the anode.Input would comprise air at the cathode.

In a preferred embodiment the devices of this invention are contemplatedto have at least 100 mW/cm² at 600-650° C. for a unit cell solid oxidefuel cell and preferably at least 200 mW/cm². The SOFC stack hasprojected power densities ranging from 0.8 kW/liter (@200 mW/cm2) to1.75 kW/liter (@400 mW/cm2) or more, and can be assembled simply bycombining the unit cells, without introducing significant additionalsealing or manifold difficulties. The invention contemplates that thisperformance will be achieved with fuel/oxidant combinations of (H₂,H₂O)/air and reformed hydrocarbon/air, but any fuels may be used. Thepresent invention contemplates that the fuel cells disclosed herein mayalso be run on fuels besides hydrogen gas, such as alcohols, propane,butane, methane, octane, and diesel and this operation is well known tothose with skill in the art. Anode gas recycle is also contemplated.

The dimensions of the cells are determined in part by the need to haveefficient edge current-collection. This is in turn determined by thein-plane conductivity of the electrodes, by non-active edge areas, etc.While not being limited to any particular dimension, calculations basedon these factors and on the known electronic resistance of the variousmaterials involved in the electrodes indicate that an approximatemaximum length for edge current collection, with a potential drop ofless than 50 mV, is between 4 and 5 cm. The projected performance of thefuel cell will therefore be sensitive to a number of geometrical factorsas well as to the intrinsic power per unit electrode area.

Insulating materials may be used in accordance with the presentinvention to insulate chipcell units from each other and/or from ahousing, or to insulate between the cell membrane and front or back wallof a holder and to adhere the cell thereto. These insulating materialsare typically ceramics, but this is not limiting. Any technique may beused to affix the cell, when used, to the holder, including brazing,adhesives and compression seals.

Brazing is the process of joining two materials by briefly melting thensolidifying an alloy. Brazing is similar to soldering except the alloysused have a melting point or melting range above 450° C. Common alloysused for brazing are based on Cu, Ni, Ag, and/or Au. Brazing to ceramicsis more difficult than brazing metals because the molten alloys do notwet oxides very well. Commercially available brazing alloys for ceramicsor metal ceramic joints often include active elements that react withthe oxide surface and promote wetting. For example, Active BrazingAlloys (ABA)® (registered trademark of Wesgo® Metals) contain additionsof elements such as titanium that promotes wetting on the ceramicsurface. Copper based brazes, such as Copper-ABA®, and silver-copperbased brazes such as Silver-ABA®, Ticusil®, Cusil-ABA® can be used forbonding and sealing the membrane in the frame as well as for bonding andsealing the insulating spacer to the frame. Gold, nickel-chromium basedbrazes can also be used in these applications.

Ceramic adhesives are used for bonding and sealing ceramics, metals,quartz, and composite materials. They often contain silicates orphosphates that, when heated, form strong bonds to the metals orceramics. Some adhesives also contain ceramic or metal particles andfibers to improve strength or improve the thermal expansion matchbetween materials.

An example of an electrically conductive adhesive is Aremco Pyro-Duct™597-A (Aremco Products, Inc, Valley Cottage, N.Y. 1098). It is asilver-filled paste suitable for sealing, bonding, and forming theelectrical contact between the cell and holder. Non-limiting examples ofcommercially available ceramic adhesives that exhibit high thermal andelectrical resistance, are Aremco Ceramabond 516, 552, 571, and 671.

In addition to the brazes and adhesives described above it is alsopossible to seal one chipcell unit to another with a compressive seal ora combination of compression and adhesive or braze. Compression can beaccomplished by simply bolting the structure together. FIG. 21 shows thesame basic chipcell design with the addition of tabs on the lowerportion that have holes for bolting the structure together. The boltsare preferably made from a terrific steel alloy and are insulated fromthe metal portion of the chipcell with Macor® or mica or glass sleeves.This allows for compression on the lower portion of the chipcell wherecell-to-cell connection is made.

The chipcell and stack design disclosed herein allows for distributingthe reactant gas in series or in parallel or both with the cells. Inother words, reactant gas enters the anode chamber of one cell, exitsthe cell, then enters the next cell in series. This type of series orcascade gas flow arrangement is know to improve the efficiency of stackswhen compared to parallel gas flow such as occurs in typical planarstack designs. Preferably, at least four units have gas flow in series.Because of the versatility of the holder and stack it is envisioned thatthere can be many series/parallel designs. A portion of the anode gasmay be recycled to improve efficiency.

EXAMPLES OF PREFERRED EMBODIMENTS Example 1

A chipcell holder was manufactured from 430 stainless steel sheet(McMaster Carr) with a thickness of 0.028 inch (0.711 cm) machined toyield a holder such as described herein with a dimension of 2.8 cm by3.6 cm by 0.21 cm thick. The size of the square frames is 2 cm by 2 cmand is approximately 0.46 cm thick. The holder consists of three parts,namely the front wall, the spacer, and the back wall or end piece asshown in detail FIG. 4A. After the sheet was cut and milled to form thedesigned parts, holes on the front piece were provided and 316 stainlesstubes (McMaster Carr; ⅛ inch OD) were inserted into them and welded toform the gas inlet and outlet. Then three parts are aligned as shown andwelded along edges to form the cell holder. FIG. 4B shows the holderparts made out of the 430 SS sheets. After connection of the 316SS tubesto the front piece, these 3 components are welded into a chipcellholder. Others were made with thicknesses of 0.457 mm and 0.15 mm. Theholders can be scaled up in size, e.g., to 5.8 cm×6.9 cm and 0.21 cmthick for housing larger membranes, e.g., 5 cm×5 cm, as shown forcomparison in FIG. 5.

Prior to mounting SOFC membranes to the holder, the holder was annealedat 750° C. for 2 h with a temperature increasing/decreasing rate of 3°C./min. For this experiment, a commercial SOFC membrane (EC TypeASC2InDEC) was cut into a square of 1.995 cm by 1.995 cm and the fourcorners were rounded with sand paper. The square SOFC membranes wererinsed in acetone three times, and the cathode side was masked into asquare of 1.5 cm by 1.5 cm.

A thin layer of silver conductive paste (Alfa Aesar) was then applied onthe masked cathode square and dried up under a heating lamp (˜60° C.),and an Ag paste was applied to the frame structures on both sides of theholder to adhere the SOFC membranes onto the respective windows in theback and front walls. The entire unit structure was then dried under theheating lamp and heated up to 700° C. for 1 h using temperature changingrate of 5° C./mm.

Silver mesh (Alfa Aesar) was then placed on top of the thin Ag layer andsilver paste was applied to cover the Silver mesh. Aremco 552 VFGceramic adhesive was drawn into a 1 ml syringe and applied to cover anygap between the holder and the mounted SOFC membranes and to bond thesilver mesh to the cell and holder. The adhesive was then cured usingthe temperature profile of 2° C./min to 93° C., dwell 2 h; 2° C./min to260° C., dwell 2 h; 2° C./min to 371° C., dwell 2 h; 2° C./min and thencooled to room temperature. A photograph of the assembled chipcell isshown in FIG. 10A.

Electrochemical performance was determined by spot welding Ag wires ontothe silver mesh, as illustrated in FIG. 10B, and the resultant unit wasplaced into a furnace. The temperature was raised from room temperatureto the test temperature with a rate of 2° C./min. A 97% H₂+3% H₂Omixture was fed from one of the 316 SS tubes to the anode chamber, andthe cathodes were left exposed to open air. Unit performance wasmeasured at 720° C. and is shown in FIG. 11. The OCV was determined tobe ˜1.02 V, and the unit was able to output ˜1.8 W of power. Since theactive area of the cathodes is 2×1.5 cm×1.5 cm=4.5 cm², thecorresponding peak power density is ˜0.4 W/cm².

Note that the holder is 2.8 cm×3.6 cm×0.21 cm, the volumetric powerdensity (VPD) is straightforwardly calculated as 0.85 kW/L. Many factorscollectively determine the VPD, among them include the size of holdersand the SOFC membranes. Trimming down the width of the window frame (thespace between the edge of the membrane and the outer edge of the holder)will increase the VPD. The VPD increases to 1.3 kW/L when the windowframe narrows down to 1 mm, a value that is reasonable for manufacture.Understandably another factor to determine the VPD is the size ofchipcell as the holder occupies a relatively significant portion ofspace in the case of small chipcell. Thus if the window frame width isheld at 1 mm, the VPD increases with the size of membranes and reaches˜2.5 kW/L for the large chipcell (5 cm×5 cm cell area with 4.75 cm×4.75cm cathode area). The VPD of chipcell stacks will be smaller than thesevalues and also strongly relies on the space between the chipcells. Forinstance, a 0.5 mm separation between the above-discussed largechipcells will decrease the VPD to 2 kW/L that is useful for practicalapplications.

Controlled Heating and Cooling

FIG. 12 displays the variation of OCV of the unit subjected to thermalcycling at different controlled heating and cooling rates. The initialOCV at 720° C. was ˜1.02V, indicating a good quality of the sealant, andremained at ˜1.03V after 57 times cycling between 220° C. and 720° C. ata rate of 3° C./min. The results shows that OCV did not degrade evenwhen the temperature change rate was increased to 10° C./min between320° C. and 720° C. for 48 times.

To examine further the stability of the chipcell during rapid thermalcycling, a second unit made in accordance with the unit described abovesubjected to thermal-shock treatment that resulted from the directremoval of the unit from hot furnace to ambient temperature and viceversa. It allows fast insertion or removal of the chipcell from thefurnace held at high temperature. The temperature change profile duringthe thermal shock procedure was recorded and displayed in FIG. 13, withthe thermocouple affixed to the chipcell. Initial heating rates are over1000° C./min. FIG. 13 shows that the chipcell sustained a heating from29° C. to 700° C. in 3.6 mins and it takes −3.8 mins to cool from 708°C. to 100° C. The OCV measured at 708° C. is summarized in FIG. 14, andit is 1.07V initially and maintains values over 1.06V even after 400times thermal shock cycles.

2-Cell Butterfly Stack

Single chipcell holders were made following the above procedure exceptthat only one of two holes was welded with a stainless steel tube.Macor™ (McMaster Carr, 0.125″ thick) was selected as the spacer toconnect the chipcells, and was machined into 9 mm*11 mm blocks and a3/16″ hole was bored through the center of the blocks. TiCuSil(68.8Ag-28.7Cu-4.5Ti; Wesgo) active brazing alloy (ABA) was tapecalendared into 0.005″ thick and then cut into 11 mm*13 mm strips wherea hole of ⅛″ was also bored through their center. Then the firstchipcell holder, ABA, MACOR, ABA, the second chipcell holder weresequentially stacked up in such a way that the centers of holes werewell aligned. The whole structure was then stabilized with a clamp, andtransferred into a brazing furnace with a typical vacuum of 1.5*10⁻⁵mmHg. The furnace was heated up to 400° C., after that the ramping ratewas set as 10° C./min and dwelled at 880° C. for 10 minutes beforecooling down to room temperature. The SOFC membranes were attached andsealed onto the manifold, following the identical procedures elaboratedin above paragraphs, to assemble the 2-unit butterfly stack as shown inFIG. 15.

Connection of Chipcells into Stacks

Since there are a variety of ways to buildup stacks based on thechipcell design, to demonstrate stack concepts two chipcells were joinedtogether via a block of MACOR using brazing techniques to form the2-cell butterfly stack displayed in FIG. 15. FIG. 16 shows a SEM imageof the joint of MACOR-brazing alloy-stainless steel, displaying goodbonding between brazing alloy and MACOR/stainless steel. In fact highmagnification observation (not shown here) reveals dense texture alongthe brazed interfaces, which suggests the gastight connection betweenchipcells is accomplished. The constructed stack was testedelectrochemically at 700° C., and the stack OCV is 2.14V that perfectlyagrees with the single chipcell OCV value of 1.07V. Obviously, bothmicroscopic and electrochemical testing results indicate that chipcellsare successfully jointed together and form a 2-cell chipcell stack. Thestack was also thermally cycled between 200° C. and 700° C. with atemperature changing rate of 10° C./min, and the variation of stack OCVwith cycling is plotted in FIG. 17. It is found that OCV stabilizesaround 2.14V, which implies a high quality seal on the membranes andconnection between the holders.

The results show that the unit SOFC according to the invention iscapable of producing a peak power density of 0.4 W/cm² or more at 720°C., and its volumetric power densities is ˜0.85 kW/L that can be raisedup without significant difficulty by optimizing holders and loaded SOFCmembranes.

The assembled unit according to the invention is extremely tolerant torapid thermal cycling, and shows no signs of gas leakage after 400 timesthermal shock treatment in which the unit bears temperature change ratesof over 1000° C./min. The unit can be heated up to 700° C. in less than3.6 minutes.

Referring to FIG. 28 and FIG. 29 it can be seen that the spacer shown inFIG. 4A is not required to form the holder, yielding two-part and single(unitary) construction. In FIG. 28 gas flow access into and out of theholder occurs through channels. Note that the two components in FIG. 28can be identical though that is not necessary. The components shown inFIG. 28 are identical and the back plate is simply the same component asthe front plate that has been turned over for assembly into the holder.A single component design simplifies manufacturing.

FIG. 28 shows window 21 and cell membrane receiving region 26 which isrecessed in plate 22. The walls may be made of any suitable materials orcombinations thereof, e.g., electrically conductive or non-conductivematerials.

FIG. 29 shows a holder made from a single component and anelectronically insulating plate 107. The solid back portion of theholder 130 of this embodiment forms the fuel cavity with the cellmembrane thus there is no window. Cell receiving region 26 is alsoprovided. The insulating plate electronically isolates the holder fromthe adjacent holder when stacked and seals the gas flow channels.

All references, patents and published patent applications disclosedherein are expressly incorporated by reference in their entireties forall purposes.

1.-21. (canceled)
 22. An electrochemical cell unit comprising: anelectrochemical cell holder and at least two electrochemical cells;wherein the at least two electrochemical cells comprises an anode, acathode, and an electrolyte; wherein the electrochemical cell holderincludes a front wall plate with at least one opening for mounting anelectrochemical cell, a back wall plate with at least one opening formounting an electrochemical cell, an electrically conductive portion ofthe holder in electrical contact with inner electrodes, and a gas outletand/or inlet adjacent to the opening; and wherein the inner electrode ofthe electrochemical cell mounted in the front wall plate and the innerelectrode of the electrochemical cell of the back wall plate are spacedapart and facing inward toward one another.
 23. The electrochemical cellunit of claim 22, wherein, the inner electrode is an anode.
 24. Theelectrochemical cell unit of claim 23, wherein the anodes are spacedapart and form the anode chamber of the electrochemical cell unit. 25.The electrochemical cell unit of claim 24, wherein the holder has flowdirecting means to direct the flow of fuel in the anode chamber.
 26. Theelectrochemical cell unit of claim 22, wherein the holder comprises anelectrically insulating portion to electronically isolate the outerelectrode from the holder.
 27. The electrochemical cell unit of claim22, wherein the electrochemical cells are mounted into the holder by aconductive or insulating seal.
 28. The electrochemical cell unit ofclaim 22, wherein the holder comprises stainless steel, ceramics,alloys, and composites.
 29. The electrical cell unit of claim 28,wherein the holder comprises ferritic stainless steel or nickel basedalloys.
 30. The electrochemical cell unit of claim 22, wherein theelectrochemical cell comprises metal or cermet support structures. 31.An electrochemical cell stack assembly comprising: a plurality ofelectrochemical cell units, wherein each electrochemical cell unitcomprises an electrochemical cell holder and at least twoelectrochemical cells; wherein the at least two electrochemical cellscomprises an anode, a cathode, and an electrolyte; wherein theelectrochemical cell older includes a front wall plate with at least oneopening for mounting an electrochemical cell, a back wall plate with atleast one opening for mounting an electrochemical cell, an electricallyconductive portion of the holder in electrical contact with innerelectrodes, and a gas outlet and/or inlet adjacent to the opening;wherein the inner electrode of the electrochemical cell mounted in thefront wall plate and the inner electrode of the electrochemical cell ofthe back wall plate are spaced apart and facing inward toward oneanother; wherein said plurality of electrochemical cell units areconnected at a portion of the periphery of the electrochemical cellholder of each electrochemical cell unit; and wherein an electronicinsulator is positioned between two adjacent electrochemical cell unitsto prevent electrical contact between inner electrodes of adjacentunits.
 32. The electrochemical cell stack assembly of claim 31, whereinthe anode gas flow arrangement comprises cascaded gas flow.
 33. Theelectrochemical cell stack assembly of claim 31, wherein the pluralityof electrochemical cell units are connected in electrical series andparallel.
 34. The electrochemical cell stack assembly of claim 31,wherein the electrochemical cell units are sealed to one another with acompressive seal, adhesive, braze or a combination of compression andadhesive or braze.
 35. An electrochemical cell stack assemblycomprising: a plurality of electrochemical cell units, wherein eachelectrochemical cell unit comprises an electrochemical cell holder andat least one electrochemical cell; wherein the at least oneelectrochemical cell comprises an anode, a cathode, an electrolyte, anda metal support structure; wherein the electrochemical cell holderincludes a back wall plate with an electrochemical cell receivingregion, an electrically conductive portion of the holder in electricalcontact with inner electrode, and a gas outlet and/or inlet adjacent tothe electrochemical cell receiving region; wherein said plurality ofelectrochemical cell units are connected at a portion of the peripheryof the electrochemical cell holder of each electrochemical cell unit;and wherein an electronic insulator is positioned between two units toprevent electrical contact between inner electrodes of adjacent units.36. The electrochemical cell stack assembly of claim 35 wherein theelectrochemical cell holder comprises ferritic stainless steel.
 37. Theelectrochemical cell stack assembly of claim 35 wherein the metalsupport structure for the electrochemical cell comprises ferriticstainless steel.
 38. The electrochemical cell stack assembly of claim35, wherein the anode gas flow arrangement comprises cascaded gas flow.39. The electrochemical cell stack assembly of claim 35, wherein theplurality of electrochemical cell units are connected in electricalseries and parallel.